A silver halide grain containing two or more parallel twin planes has a tabular form (hereinafter referred to as a "tabular-grain"). The tabular grain has the following photographic characteristics:
1) the tabular grain has a large surface area ratio to the volume (hereinafter referred to as a "specific surface area") and therefore a large amount of a sensitizing dye can be adsorbed on the surface thereof. As a result, the tabular grain exhibits a relatively high spectral sensitization sensitivity; PA1 2) when an emulsion containing tabular grains is coated and dried, the grains are arrayed in parallel to the support surface and the light scattering due to grains is reduced to thereby improve sharpness and resolution; further, this array of grains provides advantageousness in that the thickness of a coating layer can be reduced and thereby the sharpness can be improved; PA1 3) because of the large specific surface area, the development rate can be increased; and PA1 4) the tabular grain has a strong covering power, and therefore, savings of silver can be attained. PA1 (1) a method for producing a light-sensitive silver halide photographic emulsion, comprising: (a) a step of forming silver halide grain nuclei containing twin grain nuclei in a dispersion medium solution; (b) a step of ripening the grain nuclei to preferentially remain tabular grain nuclei; and (c) a step of growing the tabular grain nuclei into tabular grains to form a tabular silver halide grain; wherein in step (a), a silver halide nucleus having a chloride content of 10 mol % or more based on the amount of silver contained in the nucleus is formed, and the tabular silver halide grain obtained through steps (a), (b) and (c) has a Br content of 50 mol % or more based on the total silver amount. PA1 (2) the method for producing a light-sensitive silver halide photographic emulsion as described in (1), wherein the dispersion medium contains gelatin having at least one carboxyl group (--COOH group) newly introduced at the time of chemical modification of the amino group (--NH.sub.2 group) in the gelatin; PA1 (3) the method for producing a light-sensitive silver halide photographic emulsion as described in (1) or (2), wherein the dispersion medium contains at least one polymer having a repeating unit represented by formula (1): EQU --(R--O).sub.n -- (1) PA1 wherein R represents an alkylene group having from 2 to 10 carbon atoms, and n represents an average number of repeating units ranging from 4 to 200; PA1 (4) the method for producing a light-sensitive silver halide photographic emulsion as described in (3), wherein the polymer having a repeating unit represented by formula (1) is at least one polymer selected from a vinyl polymer having at least one monomer represented by formula (2) as a constituent component and a polyurethane represented by formula (3): ##STR1## wherein R represents an alkylene group having from 2 to 10 carbon atoms; n represents an average value of the repeating units ranging from 4 to 200; R.sup.1 represents a hydrogen atom or a lower alkyl group; R.sup.2 represents a monovalent substituent; L represents a divalent linking group; R.sup.3 and R.sup.4 each represents an alkylene group having from 1 to 20 carbon atoms, a phenylene group having from 6 to 20 carbon atoms or an aralkylene group having from 7 to 20 carbon atoms; x, y and z each represents a weight percentage of respective components, and x is from 1 to 70, y is from 1 to 70, and z is from 20 to 70, provided that x+y+z is 100; PA1 (5) the method for producing a light-sensitive silver halide photographic emulsion as described in (3), wherein the polymer having a repeating unit represented by formula (1) has a polyalkylene oxide block polymer component represented by formula (4) or (5): ##STR2## wherein R.sup.5 represents a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms or an aryl group having from 6 to 10 carbon atoms; n represents an integer of from 1 to 10, provided that when n is 1, R.sup.5 is not a hydrogen atom; R.sup.6 represents a hydrogen atom or a lower alkyl group having 4 or less carbon atoms, substituted by a hydrophilic group; and x and y each represents a number of repeating units (number average polymerization degree); PA1 (6) the method for producing a light-sensitive silver halide photographic emulsion as described in (1), (2), (3), (4) or (5), wherein a mixing vessel is provided outside a reaction vessel in which nucleation in step (a) and/or grain growth in step (c) are performed, an aqueous solution of water-soluble silver salt and an aqueous solution of water-soluble halogen salt are supplied to and mixed in the mixing vessel to form silver halide fine grains, and said fine grains are immediately supplied to said reaction vessel to effect nucleation and/or grain growth of silver halide grains in the reaction vessel; PA1 (7) a method for producing a light-sensitive silver halide photographic emulsion as described in (6), wherein the mixing apparatus comprises a closed type stirring tank equipped with a predetermined number of feeding ports for feeding said solutions to be added and stirred and a discharging port for discharging the silver halide fine grain emulsion produced after completion of the stirring process, and a stirring means for controlling the stirring condition of the solution in said stirring tank by rotation-driving at least one pair stirring blade having no rotary shaft protruding the wall of the stirring tank within the stirring tank; PA1 (8) the method for producing a light-sensitive silver halide photographic emulsion as described in (6), wherein the mixing apparatus comprises a closed type stirring tank equipped with a predetermined number of feeding ports for feeding the solutions to be added and stirred and a discharging port for discharging a silver halide fine grain emulsion produced after completion of the stirring process, and a stirring means for controlling the stirring condition of the solution in the stirring tank by rotation-driving a pair stirring blade within the stirring tank, the stirring is performed by two or more rotation-driving pair stirring blades within the stirring tank, and at least two stirring blades are disposed at opposed positions with a spacing in the tank and rotation-driven in the converse directions; and PA1 (9) the method for producing a light-sensitive silver halide photographic emulsion as described in (1), (2), (3), (4), (5), (6), (7) or (8), wherein the dispersion medium solution immediately before step (b) or step (c) is adjusted to have an ionic strength of at least 0.2 by the ion other than halogen ion.
Thus, the tabular grain has many advantageous properties and therefore, has been conventionally used for commercially available high-sensitivity light-sensitive materials.
JP-A-58-113926 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-58-113927 and JP-A-58-113928 discloses an emulsion grain having an aspect ratio of 8 or more. The term "aspect ratio" as used herein means a ratio of the diameter to the thickness of a tabular grain. The term "diameter of a grain" as used herein means a diameter of a corresponding circle having an area equal to the projected area of a tabular grain (hereinafter referred to as a "projected area diameter"). The thickness is shown by a distance between two parallel main planes constituting a tabular grain.
As the aspect ratio is larger, the tabular grain has a larger specific surface area and the advantageous properties of the tabular grain are more extensively used. In order to increase the aspect ratio, various attempts have been made to reduce the thickness of the tabular grain. JP-B-5-12696 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method for preparing a tabular grain having a small thickness using gelatin after invalidation of the methionine groups in the gelatin by hydrogen peroxide or the like, as a dispersion medium. JP-A-8-82883 discloses a method for preparing a thin tabular grain using gelatin after invalidation of the amino groups and methionine groups, as a dispersion medium. Further, U.S. Pat. No. 5,380,642 and JP-A-8-292508 disclose a method for preparing a thin tabular grain using a synthetic polymer as a dispersion medium.
Hitherto, various attempts have been made to achieve monodispersion of tabular grains and several techniques therefor are disclosed in, for example, JP-A-52-153428, JP-A-55-142329, JP-A-51-39027, JP-A-61-112142 and French Patent 2,534,036. Also, JP-A-63-11928, JP-A-63-151618 and JP-A-2-838 disclose monodisperse tabular grains containing hexagonal tabular grains. It is described in their publications that the hexagonal tabular grain is different from the triangular tabular grain, and is a monodisperse tabular grain where the tabular grain having two parallel twin planes occupies 99.7% of the entire projected area of the tabular grain and has the coefficient of variation in the circle-corresponding diameter of 10.1%. However, the tabular grains having a small thickness and a large aspect ratio cause a problem that the projected area diameter has a broad distribution and a monodisperse emulsion can be difficultly obtained.
On the other hand, U.S. Pat. Nos. 5,147,771, 5,171,659, 5,147,772, and 5,147,773 and European Patent 514,742A disclose a method for preparing monodisperse tabular grains by allowing the presence of a polyalkylene oxide block copolymer at the nucleation, where the coefficient of variation in the circle-corresponding diameter of the monodisperse tabular grains is 4.7%. JP-A-7-28183 and JP-A-7-98482 also disclose a method for preparing monodisperse tabular grains using a synthetic polymer. These techniques have successfully realized small thickness and excellent monodispersibility in the AgBr system, but in the AgBrI system, it is still difficult to achieve both the monodispersibility and decrease of thickness of tabular grains.
To overcome this problem, in Japanese Patent Application No. 8-308123, two or more carboxyl groups are introduced at the chemical modification of the amino group in the gelatin and thereby monodisperse tabular grains having a small thickness can be obtained also in the AgBrI system.
However, as the iodide content increases, the monodispersibility is worsened. In particular, when the iodide content is 5 mol % or more based on the total silver amount, it is difficult to obtain monodisperse tabular grains having a small thickness. If the grain formation is performed at a low pBr so as to reduce the thickness, the dispersibility is worsened and monodisperse tabular grains cannot be obtained.